Recently, as a power-generating equipment, a fuel cell is known, that converts energy from the fuel directly into electricity (for example, see Patent Document 1). A fuel cell is an equipment which has a cathode and a anode disposed sandwiching an electrolyte layer of thin films, and continuously generates electric power by supplying supplementing anode active material (usually hydrogen) to the anode and oxygen and the like in the air as cathode active material to the cathode to cause electrochemical reaction. A fuel cell differs from conventional batteries in that the fuel cell can continuously supply cathode and anode agents.
Currently, no such a home fuel cell system exists, which has a self-sustaining operation function in the event of power failure. However, in recent years, a self-sustaining operation function in case of power failure is in increasing demand. In response to such demand, in order to put in practical use a fuel cell power generation system which is operable being connected to the power grid at normal times or disconnected with the grid on self-sustaining operation in an emergency, or, in order to put in practice a fuel cell power generation system operable in self sustaining manner being disconnected with the grid, it is required to overcome the challenges of load following capability.
The rate of electrochemical reaction occurring in the fuel cell largely depends on temperature of the stack (reaction temperature), and the rate increases as the temperature arises. Current that can be extracted from the fuel cell is constrained by the reaction rate. The reaction process occurring in a fuel cell system is an exothermic process in total, and the reaction temperature depends on the calorific value. The calorific value becomes large as large power generation output is obtained, and the calorific value becomes small as small power generation output is obtained. Therefore, when loads to the fuel cell is small (the current is small), the power generation is lowered, and the reaction rate and reaction temperature are lowered.
With the reaction temperature being not high enough, even if the load increases rapidly, the temperature does not immediately arise. As a result, the reaction rate remaining low, current is limited and thus not able to immediately follow the load. For solid oxide fuel cell of 0.5-1 kW output, it takes few minutes to tens of minutes before the stack temperature arises high enough to follow the load. The diffusion rate of the active material used in the reaction is also dependent on the reaction rate.
With a grid-interconnected fuel cell system, load following is compensated by supplemental power received from the grid. In that case, lack of load following capability does not cause problem. However, with a the fuel cell system operable independently from the grid, or a system operable on self-sustaining operation in case of power failure, load following cannot be compensated and thus causing a problem.
Therefore, in addition to the technology for improving the load following capability of a fuel cell, a system is proposed in which the load following of the fuel system is compensated by discharge from the power-storing device having a storage battery which is equipped to the system (for example, see Patent Document 2).